The present invention relates to brakes used on, for example, commercial truck or trailer axles, and in particular to automatic slack adjusters which eliminate excess motion in a brake actuator mechanism used to apply the brake.
Over the life of the brake linings of a brake, such as a pneumatic drum brake used on commercial vehicle axles, as the brake's friction linings wear, the clearance between the brake linings and their respective friction surfaces (for example, the inner surface of a brake drum) increases. This increasing clearance requires an ever-increasing range of motion from the brake actuator mechanism to move the brake linings from their rest position to the point at which the linings contact the friction surface.
It has become commonplace to include an automatic slack adjuster in the mechanical path between the brake actuator and the brake linings so as to eliminate excess lining travel slack as the brake linings wear. Such adjusters typically are: (i) located on a portion of a brake camshaft which is outside of the brake (typically splined to the camshaft); and (ii) coupled to a pushrod of a brake actuator such that when the brake actuator push rod is extended or retracted, the slack adjuster rotates about the longitudinal axis of the brake camshaft. Thus, by extending or retracting the brake actuator pushrod, the slack adjuster causes the brake camshaft to rotate about its longitudinal axis, which in turn rotates a brake actuation cam affixed to the end of the brake camshaft located within the drum brake. The rotation of the cam either presses the brake linings into engagement with the brake drum inner friction surface or allows the brake linings to withdraw radially inward, away from the friction surface. Because the brake camshaft is used to rotate the cam which presses the brake linings radially outward, the brake camshaft is also known as the brake cam shaft.
Automatic slack adjusters are typically designed to transmit brake actuator force to the brake camshaft in the brake application direction with no relative motion between the adjuster and the brake camshaft. When the brake actuation force is withdrawn, if there is greater than desired distance between the brake linings and the brake drum friction surface, the slack adjuster is permitted to rotate relative to the brake camshaft an angular distance sufficient to remove some or all of this undesired slack, i.e., limiting the distance the brake linings withdraw from the brake drum friction surface so that the lining-drum clearance is maintained at a desired minimum.
In many automatic slack adjusters, a one-way clutch is used to accomplish the rotary adjusting movement, with a worm shaft located in the adjuster turning a worm gear coupled to brake camshaft. When the brake actuator pushrod is retracted, the worm shaft rotates about is longitudinal axis, causing the worm shaft and worm gear to move relative to one another in a circumferential direction about the circumference of the worm gear. This relative movement of the worm shaft and gear creates corresponding relative motion between the slack adjuster body and the brake camshaft. As a result, when the brake actuator pushrod returns to its rest position the brake camshaft does not return to its original rest position. Instead, the brake camshaft only rotates through a smaller angle to a new rest position. The brake application cam thus stops in a corresponding new rest position at which the brake linings are maintained closer to the brake drum friction surface. Because the rotation of the slack adjuster relative to the brake camshaft results in reduction of brake lining clearance in the new rest position, the automatic slack adjuster compensates for brake lining wear.
The one-way clutch mechanism employed in such automatic slack adjusters is usually one of: a wrap spring clutch arranged on an internal diameter of the adjuster, where the spring drives in the unwind direction (an example is the Model AA1 by Haldex Commercial Vehicle Systems, Kansas City Mo. or Model SB7 by Swedish Brake Technology Landskrona Sweden; a wrap spring clutch arranged on an external diameter, where the wrap spring drives in the wind-up direction (an example is the Model ASA-5® slack adjuster offered by Bendix Spicer Foundation Brake LLC of Elyria, Ohio); or a saw-tooth dog clutch, arranged such that saw-tooth shaped teeth engage a corresponding teeth surface in one direction, and push away from each other to slip past in the other direction. The latter arrangement is a preferred mechanism, because it employs positive drive in place of friction employed by the wrap spring type. Automatic Slack adjusters using the saw-tooth dog clutch typically have located the saw-tooth dog clutch coaxially with the adjuster's worm shaft (an example is offered by Madras Engineering Industries of India); or coaxially with the adjuster screw (an example is the Model S-ABA design offered by Haldex Commercial Vehicle Systems of Kansas City Mo.).
Adjusters with wrap spring-type clutches, particularly the internal diameter wrap spring type, have the disadvantage of reliance on friction resulting in randomly varying slip prior to engagement. Consequently they require expensive lubricants (i.e. costly high performance specialty greases) to ensure reliable adjustment. Previous saw-tooth dog clutches tend to be complicated and expensive to manufacture.
In view of the foregoing, it is an objective of the present invention to provide an improved automatic slack adjuster with the one-way clutch built within the anchor unit of the adjuster, in a manner which reduces cost and eases manufacture.
In addressing these and other objectives, the present invention provides a solution to the problems of the prior art with a design in which a saw-tooth dog clutch is built within the reference unit or anchor unit of an automatic slack adjuster which is integrated with the side cover of the automatic slack adjuster body. In effect, the reference gear wheel of the unit is split into two parts with co-axial restraint, enabling the design of a radial engagement dog-clutch. This novel arrangement also permits finer slack adjustments by use of a larger number of teeth on the anchor unit, thereby permitting finer, more accurate and consistent movement of the brake camshaft per unit angle of slack adjuster motion. This arrangement further allows for simplified manufacture because the saw-tooth dog clutch may be dovetailed into an existing flaring operation, lowering the cost of manufacture because the dog teeth are located on an internal diameter of a component of the anchor unit in a position where they can be broached at reduced cost. Further, previous designs of saw-tooth clutch have the teeth on the face of a wheel which have to be cut by milling or a similar process. The arrangement provides substantially increased torque capacity to overcome the friction between the worm gear and the body of the adjuster, which is a major factor affecting the functional performance of the adjuster. This provides the opportunity to further reduce cost by permitting use of lubricants which are less expensive than the previously used low friction-coefficient greases.
In a preferred embodiment of the present invention, the reference gear wheel is split into two parts, with co-axial restraint, thereby enabling the use of a radial engagement dog clutch.
The improvements of the present invention are made more clear in comparison with examples of previous slack adjusters. For example, U.S. Pat. No. 4,621,714 shows a slack adjuster with a rotating clutch and opposing teeth, in which the clutch engages teeth on an adjustment member and, because of the tooth shape, is only allowed to rotate counterclockwise. However, in this arrangement, the toothed clutch engagement is external at the adjuster screw, not within the center gear on the anchor pin as in the present invention.
U.S. Pat. No. 7,198,138 shows a worm gear with an integrated ratchet cylinder to perform the one-way clutch function. The present invention achieves further manufacturing advantages and finer adjustment by incorporating the clutch into the anchor pin portion and not the worm gear.
U.S. Pat. No. 3,444,758 shows a slack adjuster where the worm wheel and worm gear teeth have asymmetrical profiles. The profile prevents large braking forces from wedging the worm gear and worm wheel away from each other. The toothed engagement is external to the gear, not internal as in the present invention.
U.S. Pat. No. 3,901,357 shows an automatic slack adjuster which also allows externally-accessible manual adjustments through its arrangement of springs. The present invention does not provide access to the clutch for manual adjustments.
U.S. Pat. No. 4,561,523 shows an automatic slack adjuster that includes both a back-off sleeve and adjustment shaft which permit the slack adjuster to maintain brake clearance by either increasing or decreasing the distance between the drum and the lining. Unlike the present invention, the clutch is still part of the adjuster screw mechanism.
U.S. Pat. No. 4,718,522 shows an automatic slack adjuster with a one way coupling in the form of a wrap spring on the worm shaft. The present invention does not use a wrap spring.
U.S. Patent Publication No. US2007/0012529 shows a toothed gear portion where the gears are made from ceramic material to improve wear characteristics, but does not disclose any of the adjuster arrangements of the present invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.